Potential Direct and Diffuse Radiation Estimations (ANRAD)

Solar radiation is ultimately the driver of virtually all ecological and atmospheric systems. This study uses a model to compute the potential clear-sky radiation receipt on the slopes of the H. J. Andrews Experimental Forest Long-Term Ecological Research site in the foothills of the southern Cascade mountains of central Oregon. A comprehensive review of available modeling methods for solar radiation in complex terrain is provided. The method developed by Williams is selected and applied to the forest area for the times of the solstices and equinox. It is also applied at mid-month times in January, February, April, and May in order to completely characterize the seasonal change of potential radiation at the location. The method uses an 82 x 111 point grid with a 120 m spacing interval. Resulting maps reveal areas of the Forest with extremely steep gradients of potential radiation. These steep gradients have higher absolute values in summer compared to winter. The south-facing slopes which have the highest potential radiation values tend to be at the highest elevations. There are places which receive no direct radiation as far into the year as February. Standard deviation values of potential radiation across the Andrews show the maximum spatial variability to occur in February. There is a decrease in the ratio of diffuse to direct plus diffuse potential radiation from 0.66 at Dec. 21 to 0.23 at June 21. It seems that Lookout Creek approximately divides the Andrews Forest into an area of relatively high potential radiation to the north of the Creek and relatively lower potential radiation values to the south of the Creek. The results are also discussed in relation to spatial distributions of the values of other biophysical variables available on the Andrews Geographic Information System. Potential radiation values seem to be associated with the spatial distributions shown on the data layers of debris flows and predominant tree
species zones. A comprehensive series of appendices documents the procedures used so they that they can be employed to other parts of the forests of the Pacific Northwest and in other areas of complex terrain. Digital versions of the input data, program codes, output results, and other relevant material are also provided.